The present invention relates generally to power supply regulators, and more particularly to deactivating power MOSFETs when power supply regulators are lightly loaded.
Power supply regulators receive an input power supply at a first voltage and convert it to a regulated output power supply having a second voltage. It is desirable to perform this conversion efficiently, particularly in mobile applications where improved conversion efficiency results in longer battery life. That is, it is desirable to reduce the power consumed in converting the input voltage to a regulated output voltage.
Much of the power consumed in generating a regulated output voltage with a switching power supply regulator is consumed by driving power MOSFETs. These devices are typically driven with a pulse-width modulated signal, where the pulse-width is modulated by a feedback loop that tracks the regulated output voltage. In order to limit the power consumed by these MOSFETs, it is desirable to limit their size. Specifically, smaller devices having smaller gate capacitances take less power to drive.
Power is also lost in the MOSFETs themselves. When these devices conduct current, they provide a finite resistance known as RDSON. The current through the device flows through this resistance thus dissipating power. Therefore, it is also desirable to reduce the series resistance of the MOSFET transistors when they are in the on or conducting state.
Accordingly, there are rationale indicating that power MOSFET devices in a switching regulator should be large and others indicating that they should be small. However, one of these rationales becomes more important than the other under various load conditions placed on the regulator.
Thus, what is needed are circuits, methods, and apparatus that can vary the effective size of power MOSFETs under various load conditions.